(1) Field of the Invention:
This invention generally relates to a surface treatment of a metal body which is used under high-temperature and high-pressure conditions and is required to have both heat and corrosion resistance, for example, a spark plug, sensor or the like. In particular, this invention is concerned with a metal body with a heat- and corrosion-resistant surface which comprises two layers formed one over the other by plating techniques respectively.
(2) Description of the Related Art:
Low carbon steel is generally used in metal bodies used at high temperatures and pressures, for example, spark plugs and the like. In order to meet the requirement for heat and corrosion resistance, the metal surfaces have heretofore been subjected either to zinc plating and subsequent chromate finish or to nickel plating in practice.
Among such conventional metal bodies, there are metal bodies obtained by applying a zinc plating treatment to steel. Since zinc plating is lower in standard electrode potential than steel, zinc plating acts as an anode so that the zinc plating selectively undergoes corrosive oxidation. The zinc plating layer and steel however retain their electrical contact, whereby corrosion to the steel does not proceed and good corrosion resistance is hence exhibited. In the case of a substitute gas such as natural gas, sewage digestion gas or pyroligneous gas, components discharged as combustion products react with water, thereby forming a liquid whose pH is on the acid side. A metal body is therefore exposed to such an acidic liquid under high-temperature conditions. Regarding the surface treatment of a metal shell of a spark plug or the like, zinc plating allows red rust to occur in a rather short period of time no matter what chromate treatment is applied to its surface, because zinc itself is reactive with acids. It is also required to achieve dimensional reduction of plugs in order to meet the demand for engines having higher performance and a smaller size. It may hence be contemplated of reducing the diameter of a threaded portion of a plug with a view toward meeting the above requirement. Zinc which is the plating itself is however soft and hence acts as a solid lubricant. A considerable stress is therefore applied to the base material even when the torque applied is small, so that the base material is liable to breakage. Regarding heat resistance on the other hand, zinc plating develops discoloration at 300-400.degree. C. It is hence impossible to apply a so-called hot crimp structure, which involves heating and crimping of a thin wall portion of the metal shell.
On the other hand, nickel plating can exhibit superb corrosion resistance owing to the inherent properties of nickel itself provided that the surface is covered completely. Since a spark plug or the like is screwed in at its threaded portion, it is indispensable to reduce the thickness of the plating. Further, pinholes, cracks and the like occur easily and the corrosive liquid tends to penetrate through them. Nickel having a high standard electrode potential therefore acts as a cathode, so that its red rusting proceeds fast. With the foregoing in view, it may be contemplated of applying nickel plating in two or three layers or chromium plating to prevent pinholes and the like. In the former approach, the thickness of the plating cannot however be increased to any significant extent when the fitting readiness of the threaded portion is taken into consideration, and moreover the application of such additional layers of plating results in an increase in the fabrication cost. In the latter approach on the other hand, chromium plating may not be able to sufficiently follow deformations of the base material in a crimping step and may hence be separated from the base material. Regarding corrosion resistance to acidic liquid and heat resistance, chromium plating exhibits excellent properties compared to zinc plating. In addition, chromium plating also assures sufficient breaking strength for threaded engagement.